1. Field of the Invention
The present invention relates to a semiconductor device controlling an electrostatic actuator with MEMS (Micro Electro Mechanical Systems) and a method of controlling the electrostatic actuator.
2. Description of the Prior Art
Recently, MEMS has attracted increasing attention as a technology for achieving small, lightweight, low power consumption, and high-performance electronics. The MEMS is a system where minute mechanical elements and electronic circuit elements are integrated using silicon process technology.
An example structure of electrostatic type actuators using MEMS technology has been disclosed in Patent Document 1 (U.S. Pat. No. 5,578,976). In order to bring an electrostatic actuator into a closed state (where the upper electrode and the lower electrode come in contact with each other via an insulating film), a potential difference is applied between the upper electrode and the lower electrode so that such electrostatic attraction is provided between the electrodes that is greater than the elastic force of a movable unit to which the upper electrode is fixed.
As can be seen, for the electrostatic actuator in its closed state, the upper electrode and the lower electrode come in contact with each other via the insulating film and a larger capacitance is provided there between than in the opened state. At this moment, charges can be injected and trapped into the insulating film using an FN (Fowler-Nordheim) tunnel or a Poole-Frenkel mechanism. This phenomenon is expressed as dielectric charging in electrostatic type actuators.
When the amount of charges trapped into the insulating film due to the dielectric charging becomes larger than a certain value, the upper electrode is attracted toward the charges in the insulating film even if the potential difference between the upper electrode and the lower electrode is set to 0V. Accordingly, the electrostatic actuator cannot be changed from its closed state to opened state. This phenomenon is expressed as stiction due to dielectric charging.
One of means for avoiding such stiction has been described in, e.g., Non-Patent Document 1 (G. M. Rebeiz, “RF MEMS Theory, Design, and Technology,” Wiley-Interscience, 2003, pp. 190-191). It is difficult, however, to eliminate charges trapped into the insulating film and completely exclude stiction. While a predetermined voltage is generally applied between the upper electrode and the lower electrode for a predetermined period of time to bring the electrostatic actuator into its closed state, it is necessary to apply sufficiently high voltage for accommodating manufacturing variations of the electrostatic actuator.
In addition, it is also necessary to apply voltage for a sufficiently long period of time. The higher the applied voltage and the longer the time applied voltage becomes, the more rapid growth will be provided in the dielectric charging. Therefore, there is a need for a semiconductor device that may apply only a minimum voltage to an electrostatic actuator for a minimum period of time that is necessary for the electrostatic actuator to transition to its closed state, thereby achieving reduced power consumption, increased operating speed, and improved reliability.